1. Field of the Invention
The invention relates to a process for producing components or semi-finished products which contain intermetallic titanium aluminide alloys, and to components producible by means of the process.
2. Discussion of Prior Art
Components or semi-finished products made from intermetallic titanium aluminide alloys of conventional type, as are known in the prior art, if produced by means of conventional production processes, have drawbacks or deficits in terms of their strength, their creep properties and their ability to withstand high temperatures which are caused by the particular metallurgical features of these materials.
Titanium aluminide alloys which are of industrial importance contain 40 to 50 At. % of aluminium and up to 15 At. % of other elements, for example Nb, Cr, Si, B and C, remainder titanium. Alloys of this type are composed of γ (TiAl) as majority phase and α2 (Ti3Al) as minority phase. Further minority phases may also be present depending on the alloy composition and heat treatment. Examples of industrial titanium aluminide alloys include the following (details in At. %):
Ti-48Al-2Cr-2Nb
Ti-47Al-1Cr-1Nb-0.5B
Ti-44Al-4Nb-4Zr-0.5Si
Ti-45Al-10Nb-0.2B-0.2C.
Alloys of this type are generally distinguished by a low density, high modulus of elasticity, good strength and good resistance to oxidation. On account of these unusual properties, the titanium aluminide alloys are of considerable interest for applications in high-temperature technologies. However, an obstacle to the industrial use of the alloys is the high brittleness, which remains up to very high temperatures. Therefore, material defects or even inhomogeneities in the microstructure have an extremely disadvantageous effect on the strength and reliability of the components produced from these alloys. It has not hitherto been possible to significantly improve the low brittleness and tolerance of damage of the titanium aluminide alloys which is predetermined by the nature of the intermetallic phases by means of alloying effects. Therefore, development work aimed at suitable processes for producing titanium aluminide alloys have been concentrated on identifying process parameters for conventional metallurgical processes, such as for example casting or hot-forming, which allow very fine and chemically and structurally homogeneous microstructures to be established. On the one hand, this means that the potential for high-temperature properties cannot be fully utilized, since the fine microstructures which are set, by way of example, reduce the creep strength and toughness of the alloys. Secondly, the mechanical properties achieved in certain components are restricted by the fact that the microstructures inevitably are not fully homogeneous across the component cross sections. The reason for this is the microstructure morphology is often dependent on the local component cross section which, for example in the case of production by forging, determines the local degree of deformation, or in the case of production by casting processes determines the local cooling rate.
Like other intermetallic phases, the majority phase γ (TiAl), which is present in γ-titanium aluminide alloys, on account of its crystal structure, has considerable anisotropies in, for example, the elastic or plastic properties. Moreover, the lamellar microstructures which are preferentially established in titanium aluminide alloys boost the anisotropy of the mechanical properties. Therefore, in components certain crystal orientations of the grains are to be avoided as far as possible by the production process. However, the texture of components with greatly varying cross sections and also the microstructure can only be controlled to a limited extent over the component cross section as a whole, which in turn means that the full potential of the properties cannot be exploited.
Currently, titanium aluminide alloys can already be supplied in all product forms which are standard in metallurgy, including castings, deformed semi-finished products and powders.
The casting of titanium aluminide alloys is a relatively inexpensive production process and is suitable in particular for the production of components with a complex geometry. However, the technique is highly complex on account of the high melting point of approx. 1460° C. and the strong reactivity of titanium aluminide alloys. The mold-filling properties of titanium aluminide alloys are limited. Therefore, special casting techniques, for example centrifugal casting, are required for the production of finely shaped components. Phase transformations and ordering reactions which lead to inevitable segregation of the alloying elements and to a very pronounced cast texture occur during the solidification and further cooling of the molten material. The microstructure formed during the solidification depends on the cooling rate and can therefore vary with the wall thickness of the component. Voids and pores often occur in castings. These quantity deficits which have been listed above increase as the component size grows and cannot be tolerated for many applications.
As in the case of conventional materials, deformation technologies, such as forging or extrusion, are used to consolidate and refine the chemically and structurally very inhomogeneous castings. The improvement to the microstructure which can thereby be achieved depends primarily on the degree of deformation which can be achieved during the deformation. In the case of titanium aluminide alloys, the extent of the deformation is greatly limited in particular by the tendency of the material towards brittle fractures. As a result, cracks preventing further deformation are often formed prematurely at the periphery of forged bodies. Therefore, the degree of deformation during forging of titanium aluminide alloys is generally limited to 80%. However, this does not allow satisfactory refining and consolidation of the microstructure to be achieved. The semi-finished products which have been deformed in this way often still have regions in which the coarse cast microstructure is still present; moreover, the microstructures are still very inhomogeneous in chemical terms. Semi-finished products of this type are of only limited use for components which are subject to high levels of load.
During extrusion, high hydrostatic compressive stresses are superimposed on the deformation, which very effectively prevents the formation of cracks. This makes it possible to achieve significantly higher degrees of deformation than during forging, with the result that the quality of the microstructure is considerably improved. However, despite the extensive deformation, there are still considerable structural and chemical inhomogeneities in extruded semi-finished products as well, and these inhomogeneities greatly restrict the use of the material for components which have any safety relevance. A particular drawback for further component manufacture is that extrusion forms very slender semi-finished products. The cross section of the workpiece is usually reduced by a ratio of 10:1 or more. Currently, castings of sufficient quality can only be produced with a diameter of up to at most 300 mm. Extrusion forms semi-finished products which are only suitable for certain forms of component. However, an extruded material is eminently suitable for subsequent deformation by forging or rolling. This secondary deformation once again significantly improves the microstructure, so that the components produced therefrom can satisfy high quality demands. On account of the very slender form of extruded semi-finished products, however, subsequent forging can only fill small component volumes and in particular it is thereby impossible to produce large-area components.
However, thin metal sheets and plates of relatively large area can be produced from extruded material by rolling. On account of the rolling deformation, these metal sheets and plates are very homogeneous in structural and chemical terms but have a considerable texture with respect to the rolling direction and are therefore anisotropic in mechanical terms.
The drawbacks of the casting and deformation technologies listed above can be avoided when using powder metallurgy production processes. In these processes, pre-alloyed titanium aluminide powders are compacted a number of times by hot isostatic pressing, for which in principle there is no restriction on the size of the compacted bodies. A further advantage of these powder metallurgy production processes is that the compacted bodies are very homogeneous in structural and chemical terms and moreover do not have any texture. The mixing of powders of different composition and different microstructure also allows the profile of the mechanical properties to be varied. Therefore, titanium aluminide semi-finished products produced by powder metallurgy at first glance appear particularly suitable for the production of large components by subsequent forging.
However, a serious drawback of powder metallurgy techniques is that atomization gas is often included in the powder particles. This atomization gas is released during subsequent deforming steps and then leads to porosity. Therefore, for components which are subject to high loads and are of relevance to safety, the use of materials produced by powder metallurgy is generally avoided.